The epithelial barrier prevents the luminal content from freely entering deep tissues. Disruption of this barrier occurs in inflammatory, infectious, ischemic, and other intestinal diseases and may contribute to the pathogenesis of these diseases. The epithelial barrier is maintained by the continuous layer of epithelial cells and the intercellular junctions, primarily the tight junction, that join them. In active IBD, tight junction dysregulation increases flux between cells while apoptosis-induced cell loss disrupts epithelial continuity. Until now, these two processes (epithelial cell death and tight junction disruption) have been considered to be independent processes. My preliminary data refute this view and show that decreased expression of the tight junction protein, occludin, either by small interfering RNA mediated knockdown in cultured intestinal epithelial cells or by gene knockout in mice, decreases TNF-induced epithelial apoptosis as well as dextran sulfate sodium-induced cell loss and, in vivo, intestinal inflammation. My separate published work shows that occludin internalization is required for cytoskeletally-mediated and TNF-induced barrier loss in vitro and vivo, and that occludin overexpression limits TNF-induced barrier loss and diarrhea. Together, these observations indicate that occludin links tight junction barrier function and epithelial apoptosis. Moreover, the data suggest that occludin internalization may be an adaptive response that promotes epithelial survival in the setting of intestinal inflammation. However, this may have negative consequences, as occludin loss promotes growth and migration of colonic tumors. Thus, occludin internalization and downregulation in inflammatory bowel disease may be maladaptive. On the basis of these observations, I developed the hypothesis that occludin distribution and expression regulate epithelial survival and pathogenesis of inflammatory bowel disease and colitis-associated cancer. To address this hypothesis, three specific aims are proposed: 1). to determine how occludin expression alters intracellular signaling pathways to promote intestinal epithelial apoptosis; 2). to define how specific occludin domains and trafficking affect intestinal epithelial apoptosis; and 3). to determine how loss of occludin expression affects colon cancer cell invasion, metastasis, and colitis-associated cancer development. Answering these questions will provide molecular insights into the contributions of occludin and the tight junction to pathogenesis of intestinal diseases, and will ultimately help us to identify novel molecular targets for disease treatment and prevention. PUBLIC HEALTH RELEVANCE: The epithelium lines the inner surface of the intestine and maintains a barrier that prevents the mixing the intestinal contents from the remainder of the body. Disruption of this barrier frequently occurs in intestinal diseases and may participate in pathogenesis. The proposed mechanistic studies will advance our understanding of the contribution of the tight junction protein occludin in epithelial barrier and intestinal disease development, which will improve human health through identification of novel therapeutical approaches.